The present invention relates to a growth substrate comprising polylactic acid. The present invention further relates to a method for preparing the growth substrate.
In the tree nursery industry and in greenhouse farming, substrate (such as potting compost) is frequently used as a medium for cultivating plants. This concerns especially the cultivation in pots under glass and the cultivation of avenue trees in containers. The substrates that are used for this mainly consist of coarser parts of broken-up peat, which is used for making the soil airy.
The use of peat in horticulture is controversial, inter alia on account of the loss of biotopes caused by the extraction of peat and the emission of carbon dioxide during extraction, transport and decomposition. In addition, usable peat, such as black peat, is becoming scarcer and scarcer. Estimates are that the supply of natural peat only suffices for another 15 years of use. Furthermore, peat is a raw material which is only renewable to a limited extent, which is an undesirable property.
Another growth substrate that is used in horticulture is rock wool. Rock wool is a very costly growth substrate, however. Another drawback of these substrates is that they may constitute a health hazard, since dust particles from these substrates accumulate in the lungs upon inhalation. Another drawback is that this material is difficult to decompose.
In horticulture there is a continuous demand for renewable growth substrates which exhibit advantageous properties as regards the growth of plants and which can be discharged in a simple manner after use.
WO 2010/028037, WO 2009/011905 and NL 1017480 disclose growth substrates that may comprise polylactic acid.
WO 2010/028037 discloses a substrate based on polymer fibres, such as polylactic acid fibres. WO 2009/011905 discloses substrates based on fibre balls. Said fibres may comprise polylactic acid.
NL 1017460 discloses a growth medium which can be based on foamed polylactic acid. The substrate according to NL 1017460 is obtained by extrusion foaming. In this process, a blowing agent is added to the extruder and the polymer is foamed directly upon exiting the extruder as a result of the pressure difference inside and outside the extruder. A drawback of extrusion foaming is that only one shape is possible and that the shape and the dimensions thereof depend on the extruder. Furthermore it is very difficult to control the porosity of the substrate to be obtained such that the required porosity or approximately the required porosity is actually achieved.